Process for determining cathodically protecting current densities



Sept.

cpRRoslolv RA TE G. A. MARSH ETAL PROCESS FOR DETERMINING CATHODICALLYPROTECTING CURRENT DENSITIES Filed June 1, 1960 I I2 I I 8 [0m 0 IO'M"*CA THODIC ANODIC APPUED CURRENT/mafia F I 6. I

FIG.4

INVENTORS GLENN A. MARSH Y EDWARD SCHASCHL A TTORNE Y United StatesPatent 3,207,678 PROCESS FOR DETERMINING CATHODICALLY PROTECTING CURRENTDENSITIES Glenn A. Marsh and Edward Schaschl, Crystal Lake, 11].,

assignors to The Pure Oil Company, Chicago, 111., a

corporation of Ohio Filed June 1, 1960, Ser. No. 33,161 9 Claims. (Cl.204-1) This invention relates to mitigating the corrosion of submergedor subterranean metal structures by means of cathodic protection. Morespecifically, it relates to a method for determining the minimumcathodic current density necessary to alTord protection in a cathodicprotection system.

Almost any metal surface exposed to soil or water can be cathodicallyprotected from corrosion. In this corrosion-prevention method a voltage,great enough to cause a certain amount of current to flow to the metalstructure to be protected and render all parts of the structurecathodic, is applied from an external source to the structure. Toevaluate the economics of the cathodic protection system, it isnecessary to determine the amount of current necessary to protect thestructure. The current flow should be no greater than the minimumrequired to provide substantially complete protection, not only from thestandpoint of useless waste of power, but also to avoid excessivedestruction of the anodic member of the electrical protection system.

The prior art describes various methods of determining the currentdensity requirements to achieve cathodic protection of a metal,especially steel, in an electrolytic environment. Several arbitrarycriteria for deciding the adequacy of the applied current have beendevised. One of the most common theories is that the applied currentdensity should be the electrochemical equivalent of the rate of freecorrosion of the structure in the corrosive environment. This theory,which is in reality an application of Faradays law, has been found toprovide a fair approximation of the actual minimum current which willprovide complete protection in selected systems. In otherelectrolyte-metal systems, it has been found that the minimum current isin fact at variance with that predictable from Faradays law, usuallybeing slightly less than that which would be predicted, especially inthe case of steel in an aqueous salt solution. The use of variouscorrection factors for estimating minimum current density has beenproposed for specific corrosion systems. Other authorities have takenthe position that minimum current density can be determined for eachcorrosion system only by trial and error experiments.

One method for determining the minimum current required to achievesubstantially complete cathodic protec tion of a steel structuredisposed in an electrolyte is described in US. Patent 2,869,003, of theinstant inventors. The method of the present invention is in part animprovement over the method described and claimed in this patent in thatit provides a simplified technique by which minimum current density canbe estimated from data obtained in a single experiment.

Accordingly, it is a primary object of this invention to provide amethod for rapidly and conveniently determining the minimum currentdensity which must be applied to a metal structure, susceptible toelectrolytic corrosion, to achieve substantially complete protection ofsaid structure. It is another object of this invention to provide amethod for rapidly evaluating and controlling cathodic protectionapplied to a corrodible, submerged structure.

In accordance with this invention, the minimum current density requiredto achieve substantially complete cathodic protection of a corrodiblemetal in an electrolyte can be 3,207,678 Patented Sept. 21, 1965determined by disposing two metal specimens in the electrolyte, causinga current of known density to flow between the specimens for a suitableperiod of time, and measuring the rate of corrosion of each specimenduring the period of current flow. From the data thus determined alinear equation expressing corrosion rate as a function of currentdensity can be obtained, and this equation can be solved to determinethe minimum current density required to protect the selected metal inthe selected electrolytic environment.

This invention will be described with reference to the drawings, ofwhich:

FIGURE 1 is a graph showing corrosion rate as a function of appliedcurrent.

FIGURE 2 is a perspective view of a corrosion probe which may be used inthe method of this invention.

FIGURES 3 and 4 are schematic drawings of circuits which may be usedwith the corrosion probe of FIG- URE 2.

When .a corrodible metal specimen in a corrosive electrolyticenvironment is made positive by applying a current from an externalsource, the corrosion rate of the specimen may increase by an amountproportional to the applied current, as is predictable from Faradayslaw. Then,

Where I is the new corrosion rate expressed as the equivalent corrosioncurrent, 1 is the rate of free corrosion of the selected specimen in theselected environment, and I, is the applied current. It will beunderstood that every corrosion rate has an equivalent galvanic current,in accordance with Faradays law. Accordingly, in performing mathematicalcalculations or in constructing graphs, corrosion rates can be convertedto their equivalent currents and all calculations made in terms ofcurrents, or, alternatively, all currents and corrosion rates can beexpressed in terms of corrosion rates, and calculations similarlyperformed. It will be understood that in this specification and theappended claims, the two methods are considered to be equivalents.Experimenters of the prior art have found that the measured corrosionrate, I, for certain metals under certain conditions was less than thesum of I and 1,. Under other more commonly occurring conditions, it hasbeen found that the measured corrosion rate, I, is greater than the sumof I and 1,.

From an inspection of the foregoing equation, it is evident that When Iis negative, and equal in magnitude to I the rate of free corrosion, theactual corrosion rate I must be zero. Some experimenters of the priorart concluded from this equation that the current necessary to achievecomplete cathodic protection of a corroding metallic structure in anelectrolytic environment is the current equivalent of the rate of freecorrosion of the structure. Since, as was explained above, in a fewinstances I is in fact less than the sum of I and I and in the moreusual circumstances I is greater than the sum of I and 1,, thecalculation of the current required to achieved complete cathodicprotection of a structure using this method was only an approximation,and subject to serious error. The workers of the prior art, being awareof this error of method, proposed various correction fac tors which maybe applied, but none of which were altogether satisfactory in that Whilethe methods that they proposed might be suitable for a specific metal ina specific environment, the method still could not be applied generallywithout the introduction of serious error.

Referring to FIGURE 1, the graph is seen to depict variations ofcorrosion rate as a function of galvanic current for a corrosion system,which may be steel in an aqueous sodium chloride solution. Point 2represents the free corrosion rate of the specimen in. the electrolyte.

Through this point pass two curves, 4 and 6. Curve 4 represents thecorrosion rate which would be predicted from Faradays law, and thiscurve passes through the point 2 since it is apparent that the specimenwill corrode at its free corrosion rate when not under the influence ofgalvanic current. Assuming that galvanic currents and corrosion rate areboth represented in terms of current, the slope of curve 4 will equal 1,in accordance with Faradays law. When a galvanic current is caused tofiow to or from the corroding specimen, curve 6 is found in fact todefine the relationship between corrosion rate and galvanic current. Theslope of this line, for systems in which the corroding specimen is notsubject to anodic polarization, is greater than one.v The workers of theprior art were aware that a curve corresponding to 6 existed, but onlyin the range of point 2 to point 8, that is, where a cathodic current isapplied to the corroding specimen. It has now been discovered that curve6 extends through the free corrosion rate point, 2, and can be plottedfor anodic currents. It has further been found that curve 6 is linearfor applied galvanic currents, both cathodic and anodic, where thesecurrents have values within the range of zero to slightly less than thecurrent equivalent of the free corrosion rate. Thus curve 6 of FIGURE 1is linear in the range of point 8 to a point somewhat beyond point 110.Points 10 and 12 are obtained for 10 milliamperes per square footcurrent densities, cathodic and anodic, respectively.

The foregoing discoveries have led to the method of this invention, andare set out to provide a basis of understanding for the application ofthe method of this invention. It will be seen from FIGURE 1 that thecurrent required to achieve complete protection of a corroding specimenin an electrolytic environment is equivalent to the free corrosion rateonly when the slope of line 6 becomes equal to 1, that is, where theactual corrosion rate varies with applied cathodic current in accordancewith Faradays law. In the majority of cases in which it is desired tocathodically protect a corrodible structure in practice, the system is,such that corrosion rate varies in accordance with applied currents asdefined by a curve such as curve 6. It is evident that in such cases theslope of the curve is greater than 1, and the current density which mustbe applied to achieve complete cathodic protection of the structure issub stantially less than the current equivalent of the free corrosionrate of the specimen in the environment under study. Somespecimen-environment combinations may exist wherein the corrosion ratewill be controlled by anodic polarization. In such instances, curve 6may have a slope of less than unity, and the curve will not be linear.The method of this invention cannot be expected to predict minimumcurrent required to achieve substantially complete cathodic protectionwith the desired degree of accuracy in such cases. Accordingly, it isintended that the method of this invention be applied only to the usualcase where the free corrosion rate is not controlled by anodicpolarization.

As has been stated, point 2 represents the free corrosion rate of acorrodible test specimen, such as steel, in a corrosive electrolyte,such as sodium chloride in an aqueous solution. When a cathodic currentof 10 milliamperes per square foot is applied to the specimen, the rateof corrosion drops more rapidly than would be predicted from Faradayslaw, and point 12 is found to represent the actual corrosion rateobserved at an applied cathodic current density of 10 milliamperes persquare foot. An equivalent anodic current is then applied to thespecimen, and point It) is determined. It is found that points 2, I2,and 10 all he along the same straight line. This method has beenextended using other current densities, such as 5 milliarnperes persquare foot or milliamperes per square foot. The points were found tofall along straight line 6 as long as the applied currents, anodic orcathodic, did not exceed about 85% of the current equivalent of the freecorrosion rate of the specimen-electrolyte combination under study. Itnow becomes apparent that if two corrodible specimens are placed in anelectrolyte, and a current of known density of suitable magnitude ispassed between the specimens for a suitable length of time, thecorrosion rates of the two specimens can be measured, and two pointssuch as 10 and 12 will be thereby determined. These points may beplotted on a graph and a straight line joining the points may beextended to its intersection with the horizontal axis representing zerocorrosion rate. The cathodic current indicated by this intercept is thatcurrent required to afford complete cathodic protection for the testspecimen-electrolyte system.

Referring to FIGURE 2, a test probe which may be used to make thenecessary experiments is depicted. Basically, the probe comprises a base20, two parallel, exposed test specimens 22 and 24, and one coatedspecimen 26. The probe is used in combination with a current-supplymeans, as shown in FIGURE 3. A bridge circuit adapted for determiningthe relative corrosion rates of the specimens is shown in FIGURE 4.

In U.S. Patent application, Serial No. 528,032, filed August 12, 1955,by the instant inventors, now abandoned, there is described atemperature-compensated, corrosion-testing probe which determinescorrosion loss during the corrosion of specimens by measurement ofresistance change occuring in the test specimen. In the basicembodiments of the corrosion-testing probe, two test specimens of themetallic material of construction under consideration are disposedwithin the corrosive environment in a suitable specimen holder whichpermits the specimens to be serially interconnected. One of thespecimens is left unprotected while the other specimen is ensheathedwith a protective coating, such as a corrosion-resistant plastic toprevent its corrosion. These specimens are serially connected and formseparate resistances in one branch of a conventional electrical bridgecircuit. This combination of resistance elements constitutes acorrosion-testing unit, or probe, and functions as a sensing element forthe complete apparatus. The remainder of the bridge network, which inits simplest form consists of a second resistance branch in parallelwith the first resistance branch, a metering instrument such as agalvanorneter connected across said resistance branches, and a powersource, is positioned outside of the corrosive environment at a pointwhich will facilitate the making of observations in the corrosion study.In the second resistance branch, a variable resistance forms the secondbridge arm opposed to the corrodible speci men exposed to the corrosiveenvironment. Instrumentation which can be used in connection with thiscorrosion-testing unit includes electrical bridge circuits such as aredescribed in US. Patent 2,824,283.

In the method of this invention a modification of the afore-describedtest probes is preferably used. In this modification, two barecorrodible specimens and one coated compensating specimen are preferablyincluded in the probe, as depicted in FIGURE 2. The circuit depicted inFIGURE 3 provides the means for passing a current of known magnitudebetween the two bare specimens. In FIGURE 3, the compensating element 26has been omitted for clarity. Elements 22 and 24 serve as anodic andcathodic electrodes and are connected through lead wires 30 and 32 topower source 34, which may be a battery, through variable resistance 36,and ammeter 38. Thus the current applied to the specimens may be variedover a wide range. It is apparent that the circuit is complete only whenelements 22 and 24 are immersed in an electrolyte, such as wet soil,water, aqueous solutions, and so forth. It is especially preferred thatelements 22 and 24 lie in parallel planes to avoid uneven corrosion ofthe specimens with resulting inaccuracies. FIGURE 4 shows the manner inwhich bare specimens 22 and 24, together with protected specimen 26, areconnected in a bridge-type measuring circuit. Specimen 26 may bepermanently connected in the bridge circuit, whereas specimens 24 and 22are adapted for connection in sequence so that the change in resistanceof each specimen may be measured individually. The second branch of thebridge is provided by potentiometer 40, and power source 42 togetherwith galvanometer 44 comprise the remainder of the network. The methodof operation of the corrosion-probe measuring circuits of theresistanceratio type is well known to the art, and accordingly will notbe further described. The corrodible specimens used in the method ofthis invention preferably are foil-like, cold-rolled, steel sectionsabout 3 inches long by /s inch Wide by 0.001 inch thick. Specimenshaivng other dimensions may be used, but very thin ribbon-like specimensare preferred, because they permit the rapid obtaining of accurate data.By using suitable current densities, preferably in the range of 50% to85% of the current equivalent of the estimated free corrosion rate ofthe test specimens, accurate data can be obtained by applying thecurrent between the specimens for a period as short as one-half hour tosix hours and then making the necessary corrosion rate measurements.

The resulting data are handled as shown in FIGURE 1 to determine theminimum current required to protect the specimen. The corrosion rates,expressed as current, of the anodic and cathodic specimens are plottedagainst applied current, as indicated by points 10 and 12, respectively.Line 6 is then drawn through the points and extended to intersect theapplied current axis. Point 8, or the axis intercept, is equivalent tothe minimum protective current. While point 2 need not be plotted usingthe method of this invention, it is evident that such a point exists,and that it can be determined, if desired, by observing the point atwhich line 6 crosses the zero ordinate of applied current. Since line 6is linear, it becomes evident that the free corrosion rate of the metalspecimens can be determined by averaging the two measured corrosionrates arithmetically.

In some cases, the free corrosion rate of the metal specimens in theelectrolytic environment under study may be known. In such cases, inaccordance with this invention, a second simplified method ofdetermining the minimum current density required to achieve completecathodic protection may be used. In this method, a corrosion probe usingtwo bare corrodible specimens, as depicted in FIGURE 3, is utilized. Nocompensating specimen is necessary. The probe is inserted in theelectrolyte and a current of suitable magnitude is passed between thetwo specimens. The two specimens are then connected in a corrosion-probebridge circuit, as depicted in FIGURE 4, one specimen being placed ineach arm of the bridge, as represented in FIGURE 4 by elements 24 and26. Since no reference specimen is used, and since both of the barespecimens will corrode to some extent, the corrosion-meter circuitindicates the magnitude of the difference in the corrosion rates of thetwo bare specimens and does so in a single meter reading, rather thanindicating the absolute corrosion rates of the two specimens separately.The difference in corrosion rates having been observed, the currentrequired to achieve cathodicprotection can be calculated from theformula,

ZI L, T where,

I is the minimum current to be determined, d is the measured differencein corrosion rates, I is the free corrosion rate, and I is the currentcaused to flow between the specimens.

In instances where the free corrosion rate of the metal is not known, itcan be readily determined by use of an electric resistance-change-typecorrosion meter. A particular advantage of the method of this inventionis that it is only necessary to cause current to flow once between twobare, corrodible metal specimens. The necessity for making a pluralityof corrosion-rate determinations at a plurality of rates of current flowis avoided. Utilizing the method of this invention it is essential thatthe two corrodible metal specimens be fabricated of identical materialsof construction, and it is preferred that these speci mens have equalsurface area. It is desirable, but not essential, that the two specimenshave the same ratio of surface area to volume, and that this ratio behigh. These conditions can readily be met by using two identicalribbon-like specimens fabricated from the same material of construction.The compensating specimen, if one is used, may similarly be fabricatedof the same material of construction and have the same dimensions as thetest specimens. This is not necessary however, provided the compensatingspecimen has a temperature resistance characteristic practicallyidentical with that of the two bare specimens, so that effectivetemperature compensation is obained. The current applied between the twobare specimens need not be of any particular value, provided that thecurrent is less than that ultimately found to be necessary to achievecomplete cathodic protection. It is preferred that the currents bemaintained as high as practical, because greater accuracy is obtainedusing this method when the applied current approximates, but is somewhatless than, the current required to provide complete cathodic protectionto the specimen. Accordingly, it is preferred that the applied currentbe in the range of 50% to 75% of the electrochemical equivalent of thefree cor rosion rate of the test specimen. The free corrosion rate canusually be estimated by one skilled in this art with reasonableexactness. Where it is found that the applied current lies in the rangeof 50% to of the current determined to provide cathodic protection, itmay be assumed that an accurate determination has been made. Where it isfound that the applied current is less than 50 of the current determinedto provide complete cathodic protection, it is desirable that theexperiment be repeated using an applied current slightly less, say 10%less, than that which was indicated would provide cathodic protection.Where the current determined to provide cathodic protection is found tobe less than the applied current, it is imperative that the experimentbe repeated using a lower current density, since the accuracy of thefirst determination is doubtful.

As a specific example of the method of this invention, a probe (asdepicted in FIGURE 2) is constructed having two bare specimens 3 incheslong, /8 of an inch wide, and 0.001 inch in thickness. The probe isexposed to a cor rosive environment comprising a 1% aqueous sodiumchloride solution containing 7 parts per million of dissolved oxygen andhaving a pH of 7. A current of 62.5 microarnperes is caused to flowbetween the two bare specimens for a period of 2 hours. The corrosionrates of the two bare specimens are then measured, using the coatedspecimen as a reference element, by means of a resistance-changecorrosion-measuring instrument. The corrosion rate of the anodicspecimen is determined to be 2.8 microinches penetration per hour whichis equivalent to 50.5 milliamperes per sq. ft. The corrosion rate of thecathodic specimen is determined to be 1.1 microinche per hour, which isequivalent to .20 milliamperes per sq. ft. Using this information, andthe known current applied between the two specimens, a graphcorresponding to that shown in FIGURE 1 is constructed. The two pointsare plotted and the straight line joining them is extended to the zerocorrosion-rate intercept. At this point, the minimum current required toachieve complete cathodic protection of the selected specimens in thecorrosive electrolyte is read from the graph-in this case 28milliamperes per sq. ft.

As another example of the method of this invention, two bare steel testspecimens are inserted in a test probe in parallel relationship asdepicted in FIGURE 3. The

probe is inserted in the electrolytic solution described in the priorexample, and a current having a density of 12 milliamperes per sq. ft.is caused to flow between the two specimens. The time of current flow is2 hours. At the expiration of this time, the two corroded specimens areconnected to an electric resistance-change corrosion meter and thedifference in the rates of corrosion of the two specimens is readdirectly, and found to be 30.5 milliamperes per sq. ft., or 1.72microinches per hour. The free corrosion rate of the same material ofconstruction in the same environment is determined by inserting aconventional electric resistance-change-type corrosion probe, having abare corrodible element fabricated from the same material as theafore-described bare elements, and the rate of corrosion of this probeis determined to be 35.6 milliamperes per sq. ft. or 2.0 mircoinches perhour. The data thus obtained are then substituted in the formula,

and the current density required to achieve complete cathodic protectionof the specimens is determined to be 28 milliamperes per square foot.

The embodiments of the invention for which a special property orprivilege is claimed are defined as follows:

1. A method for determining the minimum current density required toachieve substantially complete cathodic protection of a corrodible metalin an electrolytic environment comprising disposing a cathode and ananode consisting of two bare specimens of said metal having a high ratioof surface area to volume in spaced relationship in said environment,applying between said specimens a potential from an external source tocause a direct current flow between the said specimens, determining thecurrent density applied to each of said specimens, measuring the rate ofcorrosion of each of said specimens occurring during the period of theapplied current flow, determining from said current density and fromsaid measured corrosion rates a linear equation expressing the corrosionrate as a function of current, and solving said equation for current atzero corrosion rate to determine the minimum current capable ofachieving substantially complete cathodic protection of said metal insaid electrolytic environment.

2. A method for determining the minimum current required to achievesubstantially complete cathodic protection of a corrodible metal in anelectrolytic environment comprising determining the free corrosion rateof said metal in said environment, disposing a cathode and an anodeconsisting of two elongated bare specimens of said metal in spacedrelationship in said environment, said specimens having equal surfaceareas and a high ratio of surface area to volume, applying between saidspecimens a potential from an external source to cause a direct currentof known magnitude to flow between said specimens, determining thedifference in the rates of corrosion of said specimens during the periodof said direct current flow by measuring the change in the ratio of theresistances of said specimens occurring during said period, anddetermining the minimum current density required to 8 achievesubstantially complete cathodic protection of said metal in saidenvironment by substitution in the formula:

9 21:51., where,

I is the minimum current to be determined,

d is the measured difference in corrosion rate,

I is the free corrosion rate, and

I, is the current caused to flow between the specimens.

3. A method according to claim 8 in which the said applied current is inthe range of 50 to of the current determined to provide completecathodic protection to said metal.

4. A method according to claim 11 including the steps of disposing athird reference specimen in said environment, said reference specimenhaving a temperature-resistance characteristic similar to that of saidtwo specimens, and being insulated from the corrosive effects of saidenvironment, and determining the rates of corrosion of said twospecimens by measuring the change in the ratio of the resistances ofsaid two specimens with respect to said reference specimen, saidcorrosion occurring during the period of current flow.

5. A method according to claim 4 in which the said applied current is inthe range of 50 to 75 percent of the current determined to providecomplete cathodic protection to said metal.

6. A method according to claim 4 in which the said applied current isabout of the current determined to provide complete cathodic protectionto said metal.

7. A method according to claim 6 in which said specimens are ribbon-likeand are disposed in parallel, laterally-displaced relationship.

8. A method according to claim 2 in which the free corrosion rate ofsaid metal in said environment is determined by disposing two elongatedspecimens of said metal in said environment, one of said specimens beingbare and the other being insulated from the corrosive effects of saidenvironment, and determining the rate of change of the ratio of theresistances of said specimens as said bare specimen corrodes.

9. A method according to claim 3 in which the two bare specimens areribbon-like and are disposed in parallel, laterally-displacedrelationship in said environment.

References Cited by the Examiner UNITED STATES PATENTS 2,786,021 3/57Marsh 204 2,796,583 6/57 Marsh et al. 32471.3 2,803,797 8/57 Cowles3247l.3 2,834,858 5/58 Schaschl 32471.3 2,869,003 1/59 Marsh et al324-713 OTHER REFERENCES Evans: Metallic Corrosion Passivity &Protection, 1948, pages 256, 257, 441 and 412.

Blum et al.: Transactions of the American Electrochemical Society, vol.52, 1927, pages 403-429.

JOHN H. MACK, Primary Examiner.

SAMUEL BERNSTEIN, JOHN R. SPECK, MURRAY TILLMAN, WINSTON A. DOUGLAS,Examiners.

1. A METHOD FOR DETERMINING THE MINIMUM CURRENT DENSITY REQUIRED TOACHIEVE SUBSTANTIALLY COMPLETE CATHODIC PROTECTION OF A CORRODIBLE METALIN AN ELECTROLYTIC ENVIRONMENT COMPRISING DISPOSING A CATHODE AND ANANODE CONSISTING OF TWO BARE SPECIMENS OF SAID METAL HAVING A HIGH RATIOOF SURFACE AREA TO VOLUME IN SPACED RELATIONSHIP IN SAID ENVIRONMENT,APPLYING BETWEEN SAID SPECIMENS A POTENTIAL FROM AN EXTERNAL SOURCE TOCAUSE A DIRECT CURRENT FLOW BETWEEN THE SAID SPECIMENS, DETERMINING THECURRENT DENSITY APPLIED TO EACH OF SAID SPECIMENS, MEASURING THE RATE OFCORROSION OF EACH OF SAID SPECIMENS OCCURRING DURING THE PERIOD OF THEAPPLIED CURRENT FLOW, DETERMINING FROM SAID CURRENT DENSITY AND FROMSAID MEASURED CORROSION RATES A LINEAR EQUATION EXPRESSING THE CORROSIONRATE AS A FUNCTION OF CURRENT, AND SOLVING SAID EQUATION FOR CURRENT ATZERO CORROSION RATE TO DETERMINE THE MINIMUM CURRENT CAPABLE OFACHIEVING SUBSTANTIALLY COMPLETE CATHODIC PROTECTION OF SAID METAL INSAID ELECTROLYTIC ENVIRONMENT.